I have had several stacked antenna systems since 1997 - a six stack of
tri-banders, some three and two stacks of mono-banders, and a three stack of
OWA's on 40. I have usually been able to switch all combinations and
permutations of the stack.
Roughly 85% of the time, selecting all the antennas in the stack produced
the best signal. Usually the difference between worst and best choice is
less than 3 dB, but occasionally it is very great - 20 dB or more.
The other 15% is explained by, in no particular order:
1. Domestic contests where often antennas over 40' high are down a lot
compared to low ones. The "BIP BOP" setup sometimes helps with this, but
IMHO dedicated low antennas are better.
2. Major precipitation static makes all but the low antenna almost unusable
3. Sporadic E openings on 10 and 12 meters, like the ARRL 10M contest
4. Antipodal long path to VU on 20M, where the high antenna only is often
best
5. When running a pileup with lots of callers (like JA's on 20 or 15 during
ARRL DX) switching to a different antenna often allows you to easily
separate the 'layers' of callers and pick out one to respond to.
6. High angle multiple bounce paths - mostly 10, 15 and 20 meters in the
summer, like during the IARU contest
7. All other
Was it worth all the switching gear and extra feedlines and connectors? On a
strict dB per $ figure of merit, no. Was it interesting? Very.
73 John N5CQ
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Thomson
Sent: Wednesday, July 13, 2016 11:27 AM
To: towertalk@contesting.com
Subject: [TowerTalk] 20m stack switching question
A buddy has 3 x 20m monobanders, stacked at 172, 117, 62 ft.
Currently, all 3 are driven in phase,
via a L network. ( 50 ohm to 16.66 ohms, consisting of a shunt 317.7 pf
cap from input 50 ohm connector to chassis, then
a series .2646 uh coil. Coil goes from input connector...to the 3 x
paralleled output connectors) Its just a mess of 5 kv NPO doorknob caps
in parallel + some cu tubing. Pretty straight forward. In actuality,
325 pf was used... and slightly less coil.
The plan was to install some relays, and then be able to switch top,
middle, and bottom.... and all 3 in phase. We can pull that off
with just 3 x SPST relays...and 1 x SPDT relay.
Then we thought, why not be able to use any 2 x yagis at a time, like top +
middle... middle + bottom.... and also top and bottom.
That could be done with a 2nd L network + 1 x additional SPDT relay.
Ok, is it even worth it to be able to use any 2 yagis ?? For DX, all 3
in phase work pretty good. For local, say within 3000 miles, perhaps
the lowest yagi alone would suffice? I cant see the top and bottom
buying him anything..... vs all 3 in phase. I cant see the top and middle
outperforming all 3 in phase. That only leaves the middle and bottom..and
even that is dubious... but we have zero experience with this.
Even with the 4 x relays and just one L network box, we could still easily
select any 2 yagis..and use the 50: 16.66 ohm L network... however
the swr will not be dead flat, but it would be < 2: 1 25 / 16.66 =
1.5:1 swr.... which is still useable.
Are we wasting our time ? On a side note, the pair of 40m yagis, 180 + 89
feet, are also driven in phase.... with no provisions for top + bottom
switching. However, BOP was added to the BIP. Easily done with a
switchable 1/2 wave of coax in either leg. Then countless hrs spent
switching between
BIP...and BOP. BOP was a disaster. Only once was BOP a bit louder... and
that was on 40M during the daytime..and just one station, aprx 200 miles
away.
Looking at some notes here and there, it appears that BOP has a higher
take off angle than the lower 40M by itself...which ends up way too high.
Latest plan is to be able to switch top /bottom /both on the pair of 40m
yagis......and ditto with the pair of 15M yagis ( 127 + 79 ft) . We could
also add the BOP function, but after the 40M failed experiment, I think BOP
is a total write off.
Any thought on any of this would be greatly appreciated. BTW, all yagis
are pointed in the same direction on a rotating tower. In the original
config,
non rotable tower, tic rings were used on 40M + 20m.... then all ants could
be pointed in different directions. That came in handy at times, but the
tic rings were a constant head ache.
Jim VE7RF
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